The present invention relates to a charge-coupled device (CCD) solid-state image sensor having a built in test structure which utilizes an optical injection to measure the charge transfer efficiency for a CCD solid-state image sensor.
Many high-end imaging applications today utilize large format, area CCD image sensors. These area arrays are composed of a two-dimensional array of pixels, often called the vertical or parallel registers, that are usually transferred row by row into a single row, often called the horizontal or serial register that is used to clock out the signal. Some sensors may have more than one horizontal register. The benefits of this architecture are the high sensitivity, high charge capacity and low dark currents resulting in very large dynamic ranges. An important measure of performance for these large format imager sensors is charge transfer efficiency (CTE), which measures how completely charge is transferred along a CCD register. Brodersen, et al., in xe2x80x9cExperimental Characterization of Transfer Efficiency in Charge-Coupled Devicesxe2x80x9d, published in IEEE Transactions on Electron Devices, ED-22, No. 2, February 1975, pp. 40-46, (incorporated herein by reference) describe the fixed loss, proportional loss and nonlinear loss of charge which degrade CTE. If CTE=1, then all of the charge has been transferred from one phase to the next. The fraction of charge left behind is called the charge transfer inefficiency (CTI) and CTI=1xe2x88x92CTE. If a fraction of charge from one pixel is left behind during transfer, it will result in a loss of contrast between adjacent pixels in the image. For color sensors, it can result in a hue shift, since the charge in adjacent pixels usually represents different colors. Mixing of the charge in adjacent pixels due to CTI will result in a change in color. Good CTE in both the vertical and horizontal registers is important, however, good CTE in the horizontal register is more difficult to achieve because of the higher clock frequency, or, alternatively, the short time available to transfer charge from one phase to the next. Since many of the large format image sensors now require well over 5000 transfers in a register, CTI of less than 10xe2x88x925 (or CTE greater than 0.99999) is important to maintain signal integrity and prevent contrast loss and color errors.
CTE is typically measured by the inclusion of a xe2x80x9cfill and spillxe2x80x9d electrical injection circuit (M. F. Tompsett, IEEE Transactions on Electron Devices, ED-22, No. 6, June 1975, pp. 305-309 and W. F. Kosonocky and J. E. Carnes, RCA Review, 36, p. 566, September 1975, incorporated herein by reference) incorporated on the input end of the CCD shift register (usually the horizontal). The difficulty with electrical injection structures (additional gates and diodes added at the end of a CCD shift register where charge packets are created by xe2x80x9cclockingxe2x80x9d a charge packet into the array) is that they require adjustment for each individual die, thus making automation of testing more difficult. In addition these small gates are sensitive to electrostatic discharge (ESD); loss of an otherwise functional device can result from ESD failure of the test structure.
X-ray radiation sources, such as Fe55, provide a known and constant input level, but the additional apparatus and safety precautions inhibit its use in a production test environment. The fixed energy levels of a particular isotope also prohibit the ability of measuring CTE as a function of signal level. In addition, one of the most useful sources, Fe55, has a limited useful lifetime.
Optical injection, i.e., the use of optically generated packets, has also been used for characterization of CTE for a linear array. Herbert J. Erhardt, U.S. Pat. No. 5,369,357 of Nov. 29, 1994 incorporated herein by reference, describes an implementation for linear CCD arrays. This requires separate gate controls to transfer the optically generated packets into the CCD register for characterization of CTE. This would be difficult to implement for area arrays.
This invention incorporates special pixels in the CCD imager that enable in-situ monitoring of CTE. These special pixels include a photoactive column (or columns) at the leading and trailing edge of the device beyond the dark reference regions. This column (or columns) is referred to as a vertical optical injection column. A charge packet for the CTE measurement is xe2x80x9cinjectedxe2x80x9d from the vertical optical injection column into the horizontal register by a normal row transfer.
In order to remove optical and diffusion crosstalk components arriving from the photoactive column, adjacent scavenging columns are added that transfer charge in the opposite direction of normal vertical charge transfer. These scavenging columns collect charge generated by stray light or charge generated below the depletion region of the optically active pixels that diffuses to the adjacent scavenging columns. These scavenging columns are electrically connected to drains at the top of the array to remove any charge collected in these pixels. The scavenging columns are not electrically connected to the horizontal CCD register; thus no charge is transferred from a vertical row into the corresponding pixels of the horizontal register under the scavenging columns.
The only charge read out from the horizontal pixel that follows the pixel containing the optical injection signal must come from charge that was not transferred so that an accurate measurement of CTE can be made. The input stimulus is supplied by illuminating the sensor with light. Varying light intensity or exposure produces a transfer curve of transfer efficiency as a function of signal. For each different exposure, there will be a resultant signal level. The resulting horizontal profile can be used to calculate the transfer efficiency in the case of few transfers (leading edge) and many transfers (trailing edge). If the sensor is uniformly illuminated or if it is read out such that the same charge is transferred from each pixel of the optical injection column or columns, horizontal profiles may be averaged to improve the accuracy of the CTE calculation. Software routines within a camera can use this information to compensate for such inefficiencies.